In order to provide the transmission ratios, automatic transmissions for motor vehicles comprise several frictionally engaging shift elements, which can, for example, transfer torque under a hydraulic pressure. With the amount of the pressure, which is also known as the clutch pressure, the shift element halves are pressed against each other with an increasing force, if the shift element is disengaged in a pressureless state. As a result, the amount of torque that can be transferred by the frictionally engaging shift element, which in the following is designated as the transfer capability, increases. An increase of transfer capability is therefore understood to be an increase of the torque that can be transferred by the shift element. Transfer capabilities start when the volume of the shift element is filled and the friction surfaces of the shift element halves are in contact with each other. With the contact, torque is transferred and at the same time there is slippage of the shift element, that is, there is a dynamic friction between the shift element halves. With increasing pressure, the contact pressure of the shift element halves on each other increases, and thus also the transferable frictional torque. This applies in an analogous manner to shift elements that are actuated by a different active energy, for instance, frictionally engaging shift elements that are engaged electromechanically.
The different transmission ratio steps of an automatic transmission are created when different shift elements are engaged in specific combinations, and thereby connect, in a rotationally fixed manner, the different elements of a planetary gear set to each other or lock these elements. In order to achieve good shifting comfort during gear changes, also called gear transitions, a shift element, which is to be newly engaged, must not be pressurized abruptly, but rather the buildup of pressure must increase steadily. In order to guarantee a steady increase of the transfer capability of a frictionally engaging shift element, the pressure is increased in a defined ramp-like progression. The pressure is adjusted using an electronic transmission control unit (EGS), which outputs an electrical current as a control variable and thereby controls an electrohydraulic gearshift device (HSG), which thereupon sets a specific pressure, depending on the value of the current, by means of an electrical pressure regulator (EDS). It should be noted that the current-pressure correlation can be different for each pressure regulator due to the manufacturing tolerances of the pressure regulator.
In order to be able to reproducibly set the desired pressure ramp, it is necessary to have precise knowledge of the correlation of current, which controls the EDS, and pressure generated by the EDS.
The value of the current required for generating a specific clutch pressure can be determined from a table stored in the EGS or calculated from the desired clutch pressure using a mathematical function. The mathematical function is a polynomial, for example. Specific characteristic values, designated also as compensation data, must be known in order to define the polynomial or the table; for example, a fill current, which is issued by the EGS and conducted to the electrical pressure regulator in the hydraulic gearshift device, which then generates a fill pressure in the hydraulic system or in the shift elements. The fill pressure is attained when the shift element, for example a hydraulic clutch or brake, is filled to the extent that a specific pressure has been built up and the friction surfaces of the shift element have approached each other to the extent that the transfer of a torque begins.
The fill pressure, which can differ for each shift element within an automatic transmission depending on the geometric shape, is therefore associated with a specific value of the fill current. In this context, the experimentally determined fill pressure is identical for all shift elements of the same shape, even if these elements are disposed in different transmissions of the same type.
Transmission concepts are known that have a mechatronic system. A mechatronic system is a rigidly installed assembly that substantially comprises an HSG and an EGS. It is possible to compensate for tolerances in the current-pressure correlation of the electrical pressure regulator, for example, by means of compensation data handling, and thereby to be able to use inexpensive pressure regulators for high shift quality of new vehicles.
In contrast, in the case of separate HSG and EGS components, such compensation data handling is not possible in the event of a subsequent exchange of one or both components in the field.
A method for determining characteristics of an automatic transmission is known from the document DE19643305 A1. For this, an automatic transmission on a final test bench is shifted into the individual transmission ratio steps, wherein an input and output transmission speed and an input and output transmission torque are measured. For example, a fill time, a fill pressure, a reaction time or a friction value of the plates are determined from these measured values as characteristic values of the automatic transmission such for the clutch to be engaged during shifting. These characteristic values are then stored in a memory so that an electronic transmission control device can correct the pressure level and the time of a rapid filling pressure, of the fill pressure and the pressure level of a shift pressure based on these characteristic values. In this method, the fill pressure, for example, is determined in that, with the automatic transmission driven, the pressure level in the respective shift element is increased until the output torque exceeds a defined limit value.
A disadvantage here is that torque measurement requires expensive measurement equipment and sensors. In addition, such a measurement can occur only on a test bench using a transmission that has been removed from the vehicle, which is not service-friendly and is expensive with respect to the installation effort and costs for an application in the field or service sector.